To enable safe driving during rain, some roads have lateral slopes on their surface so that rain will not accumulate on the road surface. When a vehicle is running on such a road that has a lateral slope, the vehicle drifts towards a lower portion of the road surface. Hereinafter, this phenomenon will be called “vehicle body drifting.”
When vehicle body drifting occurs in this way, a driver's burden of driving the vehicle increases because the driver is forced to adjust the operation of the vehicle by using a steering wheel in order to drive straight without having the vehicle drift towards the lower portion of the road surface.
Consequently, vehicle body drifting restraining devices have been developed, which restrain the drifting of the vehicle body while reducing the driver's burden of driving the vehicle. For example, an electric-powered steering device is known, which increases the control of a steering assist amount when vehicle body drifting is detected, compared to when vehicle body drifting has not occurred (for example, refer to Japanese Unexamined Patent Application, First Publication No. 2007-168617).